TH96752B - High-carbon hot-rolled steel sheets and methods for producing them. - Google Patents

Abstract

What is provided are high-carbon hot-rolled steel plates assembled with B-grade steel. The steel plates exhibit excellent maintainable hardening properties, even after nitrogen-treated annealing, and excellent formability, specifically a hardness of 83 HRB or less and a total elongation of 30% or more, or even better formability, specifically a hardness of 75 HRB or less and a total elongation of 38% or more, before quenching. High carbon hot-rolled steel sheets contain C: 0.20% or more and 0.53% or less, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0050% or less, B: 0.0005% or more and 0.0050% or less, and one or more elements selected from Sb, Sn, Bi, Ge, Te, and Se in such a way that the total amount of one or more elements is 0.002% or more and 0.030% or less, and have a microstructure that includes ferrite and cementite. The cementite density in ferrite grains is 0.15 particles/micrometer² or less when C: is greater than 0.40% and 0.53% or less. The cementite density in ferrite grains is 0.10 particles/micrometer² or less when C: is 0.20% or greater and 0.40% or less.

Claims (11)

1. High-carbon hot-rolled steel plates composed of elements containing C: 0.20% or more and 0.53% or less, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0050% or less, B: 0.0005% or more and 0.0050% or less as % by mass. One or more selected elements are derived from Sb, Sn, Bi, Ge, Te, and Se in such a way that the total amount of one or more elements is 0.002% or more and 0.030% or less, and the balancing element consists of Fe and accidental impurities. The high-carbon hot-rolled steel plate has a microstructure that includes ferrite and cementite, with the cementite density in ferrite grains being 0.15. High carbon steel hot-rolled plates have a microstructure that includes ferrite and cementite, with in-grain cementite density of ferrite being 0.10 particles/micrometer² or less, hardness greater than 65 HRB and 83 HRB or less, and total elongation of 30% or more, when the C content is C: greater than 0.40% and 0.53% or less as % by mass. 2. High carbon hot-rolled steel plates according to claim 1, where the C content is C: greater than 0.40% and 0.53% or less as % by mass, and the high carbon hot-rolled steel plates have a microstructure including ferrite and cementite, with a cementite density in the ferrite grain of 0.15 particles/µm² or less, hardness greater than 65 HRB and 83 HRB or less, and total elongation of 30% or more. 3. High carbon hot-rolled steel plates according to claim 1, where the C content is C: 0.20% or greater and 0.40% or less as % by mass, and the high carbon hot-rolled steel plates have a microstructure including ferrite and cementite, with a cementite density in the ferrite grain of 0.10 particles/µm² or less, hardness greater than 65 HRB and 75 HRB or less, and total elongation of 38% or greater. 4. High carbon hot-rolled steel plates pursuant to any one of the claims 1 through 3, whereby the composition is further constitutively at least one element selected from Ni, Cr, and Mo in such a manner that the total amount of at least one element is 0.50% or less as a percentage by mass. 5. High carbon hot-rolled steel plates under any one of Requisitions 1 through 4, where the variation in HRB hardness in the width direction of the steel plate is 4% or less, and the variation in total elongation in the width direction of the steel plate is 3% or less. 6. Method for the production of high-carbon hot-rolled steel sheets, where the method involves using steel with a composition of C: 0.20% or more and 0.53% or less, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0050% or less, B: 0.0005% or more and 0.0050% or less as % by mass, one or more elements selected from Sb, Sn, Bi, Ge, Te, and Se in such a quantity manner. The inclusion of one or more elements in amounts of 0.002% or more and 0.030% or less, resulting in an equilibrium composition including Fe and accidental impurities, is achieved through coarse hot rolling, fine rolling at a fine rolling temperature of Ar3 phase transition temperature or higher and (Ar3 phase transition temperature + 90 °C) or lower, coiling at coiling temperatures of 500 °C or higher and 700 °C or lower, and annealing at an ACl phase transition temperature or lower. This process produces high-carbon hot-rolled steel sheets with a microstructure including ferrite and cementite. High-carbon hot-rolled steel plates with a microstructure including ferrite and cementite have a cementite density in the ferrite grain of 0.15 particles/micrometer² or less, a hardness greater than 65 HRB and 83 HRB or less, and a total elongation of 30% or more, when the C content in the steel is C: greater than 0.40% and 0.53% or less as a percentage by mass. Similarly, high-carbon hot-rolled steel plates with a microstructure including ferrite and cementite have a cementite density in the ferrite grain of 0.10 particles/micrometer² or less, a hardness greater than 65 HRB and 75 HRB or less, and a total elongation of 38% or more, when the C content in the steel is C: 0.20% or more and 0.40% or less as a percentage by mass. 7. A method for the production of high-carbon hot-rolled steel sheets according to claim 6, where the carbon content in the steel is C: greater than 0.40% and 0.53% or less as % by mass, and the high-carbon hot-rolled steel sheets have a microstructure including ferrite and cementite, with a cementite density in ferrite grains of 0.15 particles/µm² or less, a hardness greater than 65 HRB and 83 HRB or less, and an overall elongation of 30% or more, has been produced. 8. A method for the production of high-carbon hot-rolled steel sheets according to claim 6, where the carbon content in the steel is C: 0.20% or greater and 0.40% or less as % by mass, and high-carbon hot-rolled steel sheets with a microstructure including ferrite and cementite, where the cementite density in ferrite grains is 0.10 particles/µm² or less, hardness greater than 65 HRB and 75 HRB or less, and total elongation of 38% or greater, has been produced. 9. Methods for the production of high carbon hot-rolled steel sheets pursuant to any one of the claims 6 through 8, whereby the steel is further processed with at least one element of choice from Ni, Cr, and Mo in such a way that the total amount of at least one element is 0.50% or less as a percentage by mass. 10. Any method for the production of high carbon hot-rolled steel sheets under claims 6 through 9 where edge heaters are used in fine rolling. 11. Method for the production of high carbon hot-rolled steel sheets under claim 10, in which the difference between the temperature and the fine-rolling temperature at the center of the steel sheet in the width direction and the fine-rolling temperature at 10 mm from the edge of the steel sheet in the width direction is set to 40 °C or less by the use of edge heaters.